During the refining operation great amounts of steam are generated which to a certain extent pass out of the refiner together with the refined material, but a large part of the steam flows rearward to the chip feed. This steam flow can disturb the chip feed flow so that it becomes non-uniform, which implies that also the refining result will be non-uniform.
The chip feed usually is carried out by means of a screw or the like, which rotates at uniform speed and thereby feeds the material toward the openings in one refining disc. The feed speed then is low, normally about 2 m/s, and if, besides, the feed passageway has a small cross-section, the rearward flow of the steam can disturb the chip feed.